Different kinds of optical filters are known. Optical filters can be formed by coating a blank to form an optical filter which has different characteristics in different locations of the filter.
Examples of such filters are found in U.S. Pat. No. 5,426,576. In these optical filters, the amount of color saturation may vary based on the distance along a gradient axis. The gradient axis can be a linear gradient axis, or a circumferential gradient axis, in this patent. Also, two filters can be used together to form a cross fader.
Different kinds of coated optical filters are also known. In general, these coated optical filters may have characteristics that vary based on the amount of the coating.